Internal combustion engines disposed in vehicles are typically provided with a catalyzer, first and second oxygen (herein O.sub.2) sensors, and a control means. The catalyzer is located in an exhaust path of the internal combustion engine. The first and second O.sub.2 sensors, which serve as exhaust sensors, are positioned in the exhaust path upstream and downstream of the catalyzer respectively. The control means provides feedback control such that an air-fuel ratio achieves a desired value in accordance with first and second detection signals which are respectively sent out from the first and second O.sub.2 sensors. The internal combustion engine is thereby adapted to improve the exhaust cleanup efficiency of the catalyzer in order to reduce values of noxious exhaust components that are discharged from the internal combustion engine.
Japanese Patent Laid-Open Nos. 4-109045 and 4-116239 disclose such internal combustion engines designed for reducing the values of exhaust noxious components discharged therefrom.
The internal combustion engines disclosed in said U.S. Pat. No. 4,109,045 has first and second O.sub.2 sensors disposed respectively upstream and downstream from an exhaust cleaning means positioned in the exhaust path of the internal combustion engine. Feedback control is performed in accordance with a first detection signal from the first O.sub.2 sensor to match an air-fuel ratio with a desired value. When the desired value is changed, the above system is adapted to monitor the deterioration of the cleaning means in accordance with an ensuing response change in a second detection signal sent out from the second O.sub.2 sensor.
The internal combustion engines disclosed in said U.S. Pat. No. 4,116,239 also has first and second O.sub.2 sensors disposed respectively upstream and downstream from an exhaust cleaning means (i.e. catalyzer) positioned in the exhaust path of an internal combustion engine. Feedback control is also performed in accordance with a first detection signal from the first O.sub.2 sensor to match an air-fuel ratio with its desired value. However, the internal combustion engine further includes a deterioration-determining means and a determination-inhibiting means. The deterioration-determining means determines the deterioration of the catalyzer by comparing the output of the first O.sub.2 sensor with that of the second O.sub.2 sensor while feedback control is effected. The determination-inhibiting means inhibits the deterioration-determining means from determining the deterioration of the catalyzer if the number of updates of a feedback control learning value is less than a predetermined number.
The cleaning function of a catalyzer located in an exhaust path of an internal combustion engine typically will not significantly decrease when the catalyzer is used in a normal operating state of the internal combustion engine. However, there are cases where the catalyzer experiences poisoning caused by lead, or failures due to crude gases and the like, such as when an internal combustion engine designed for receiving lead-free gasoline as fuel is supplied with lead-containing gasoline, or when the internal combustion engine operates with an improper or malfunctioning ignition plug. Such poisoning and failures in the catalyzer cause deterioration of the catalyzer and significantly degrades its cleaning function, thus reducing the exhaust gas cleanup efficiency of the catalyzer. As a result, the degraded catalyzer cannot prevent a large quantity of uncleaned exhaust from being discharged into the air, which results in environmental pollution.
Accordingly, it is desirable that the internal combustion engine precisely measure and determine with a high degree of accuracy a deteriorated state of the catalyzer. However, if the deteriorated state of the catalyzer is determined with a low degree of accuracy, a warning is typically issued that there are abnormalities in the catalyzer regardless of whether the catalyzer is or is not operating normally. This causes inconvenience and needless confusion, and reduces reliability.
Among the catalyst deterioration-determining devices for determining a deteriorated state of the catalyzer as described above, there is one type of device which obtains a deterioration-determined value from a calculation as described below and compares it with a deterioration-set value to determine the deteriorated state of the catalyzer. The deterioration-determined value is calculated on the basis of a lean response delay time and a rich response delay time, which times respectively elapse from the beginning of a decrease and an increase in a first feedback control-corrected quantity, to a lean inversion time and a rich inversion time of a second detection signal from the second O.sub.2 sensor. The deterioration-determining value is calculated in response to rich and lean inversions of a first detection signal which is sent out from the first O.sub.2 sensor. However, as described below, there is a problem with such a device in which a cleaning ratio of the catalyzer is simulated and determined on the basis of the lean response delay time and the rich response delay time of the second O.sub.2 sensor.
As illustrated in FIGS. 15A-15F, one response delay time t1 (i.e., t1=TLR), and the other response delay time t2 (i.e., t2=TRL), of the second O.sub.2 sensor are interrelated respectively with periods T1 and T2 of a first feedback control-corrected quantity FAF, through the first O.sub.2 sensor. A first detection signal FO.sub.2, from the first O.sub.2 sensor provides a rich inversion (or a lean inversion) at the time of integration after every skip at which the first feedback control-corrected quantity FAF begins to decrease and increase. Thereafter, a second detection signal RO.sub.2, from the second O.sub.2 sensor provides a rich inversion (or a lean inversion).
A rich determination delay time DLR, elapses from a rich inversion time of the first detection signal FO.sub.2 to the beginning of a decrease in the first feedback control-corrected quantity FAF. A lean determination delay time DRL, elapses from a lean inversion time of the first detection signal FO.sub.2 to the beginning of an increase in the first feedback control-corrected quantity FAF. Next, as shown in FIGS. 15A-15F, comparison is made between the first detection signal FO.sub.2 when DLR/DRL is assumed to be, for example, 0 ms and that when DLR/DRL is assumed to be 197 ms, with reference to the period of the single first feedback control-corrected quantity FAF. It is found that in the latter case, before the beginning of decreases and increases in the first feedback control-corrected quantity, the first detection signal FO.sub.2 provides the rich inversions (or the lean inversions) earlier by 197 ms than the former case.
Accordingly, when the rich determination delay time DLR and the lean determination delay time DRL are 197 ms, the second O.sub.2 sensor responds earlier (t1&gt;t2). However, there is a problem in which a change in the determination delay time of the first O.sub.2 sensor generates a variation in the response delay time of the second O.sub.2 sensor. Further, as shown in FIGS. 16A and 16B, an air-fuel ratio at the beginning of a decrease and an increase in the corrected quantity through each skip varies with a change in the period of the first feedback control-corrected quantity FAF. Simultaneously, a catalyzer O.sub.2 storage capacity at this time is changed as well. This produces a problem of being a factor generating a fluctuation in the response delay time of the second O.sub.2 sensor.
Referring to FIGS. 10 and 11, a response delay time TDLYAV of the second O.sub.2 sensor varies with a first feedback control period TFB. Therefore, the response delay time TDLYAV must be corrected in accordance with the period TFB of first feedback control-corrected quantity at the time of measurement.
A lean response delay time TRL elapses between the beginning of a decrease in the first feedback control-corrected quantity FAF and a lean inversion time of the second RO.sub.2 signal in response to a rich inversion of the first detection signal FO.sub.2 that is sent out from the first O.sub.2 sensor. A rich response delay time TLR elapses between the beginning of an increase in the first feedback control-corrected quantity FAF and a rich inversion time of the second RO.sub.2 signal in response to a lean inversion of the first detection signal FO.sub.2. As illustrated in FIG. 14, when a value TDLY, at which it is determined that a catalyzer is deteriorated, is determined simply from an equation TDLY=(TRL+TLR).div.2, the deterioration-determined value TDLY varies with the rich determination delay time DLR and the lean determination delay time DRL of the first feedback control-corrected quantity FAF. Such a variation results in a problem in that the deterioration-determined value TDLY forms a non-linear relationship with the first feedback control period TFB. The deterioration-determined value TDLY thereby fluctuates a great deal. This prevents the cleaning rate of the catalyzer from being measured with high precision.
In order to obviate or minimize the above-described inconveniences, the present invention provides a catalyst deterioration-determining device for an internal combustion engine having first and second exhaust sensors positioned in an exhaust path of the internal combustion engine upstream and downstream respectively of the catalyzer located in the exhaust path. The internal combustion engine also has a control means for effecting feedback control to match an air-fuel ratio with a desired value in accordance with first and second detection signals from first and second exhaust sensors, respectively. The catalyst deterioration-determining device comprising a determining section of the control means which obtains a deterioration-determined value from a calculation based on a rich determination delay time, a lean determination delay time, a lean response delay time and a rich response delay time when a deteriorated state of the catalyzer is determined. The lean response delay time and the rich response delay time elapse from the beginning of a decrease and an increase in the first feedback control-corrected quantity respectively to a lean inversion time and a rich inversion time of the second detection signal. The deterioration-determined value is obtained in response to rich and lean inversions of the first detection signal, and the rich and lean determination delay times elapse from a rich inversion time and a lean inversion time of the first detection signal, respectively to the beginning of the decrease and increase in the feedback control-corrected quantity. Therefore, the determining section performs the calculation in accordance with the deterioration-determined value to determine the deteriorated state of the catalyzer.
Pursuant to the above structure of the present invention, when a deteriorated state of a catalyzer is determined, the determining section provided in the control means obtains the deterioration-determined value from calculation based on: the lean response delay time and the rich response delay time, which respectively elapse from the beginning of a decrease and an increase in the first feedback control-corrected quantity to the lean inversion time and rich inversion time of the second detection signal. The deterioration-determined value is obtained in response to the rich and lean inversions of the first detection signal; and, the rich and lean determination delay times which respectively elapse from the rich inversion time and lean inversion time of the first detection signal to the beginning of the decrease and increase in the feedback control-corrected quantity. In accordance with the deterioration-determined value, calculation is made to determine the deteriorated state of the catalyzer.
As described above, determination is made after the deterioration-determined value is obtained from calculation based on the rich determination delay time and the lean determination delay time of the first feedback control-corrected quantity in addition to the lean response delay time and the rich response delay time of the second detection signal. This method can eliminate any influence on the determination of a deteriorated state of the catalyzer due to manufacturing non-uniformities or deterioration during use of the first exhaust sensor which is located in the exhaust path upstream from the catalyzer. As a result, the deterioration-determined value can be interrelated linearly with the period of the first feedback control-corrected quantity.